In comparison to known current transducers (CT) with a ferromagnetic core, the Rogowski coil current transducers (RCCT) or sensors, exhibit higher dynamic range, lower weight and size, as well as lower production cost.
However, known RCCTs often have a moderate accuracy compared with highly accurate CT based current measurement solutions for metering applications. There have been various attempts to improve the Rogowski coil performance and associate it with advanced electronics to reach IEC60044-8 class 0.5 metering accuracy.
Known solutions for improving the accuracy of an RCCT are based on calibration techniques which provide for an interruption of the measurement or an additional reference sensor to measure the sensitivity of the Rogowski coil.
A first known solution is called “RCCT initial calibration in factory”. Here, the Rogowski coils are calibrated in the factory. Their initial sensitivity as well as their temperature dependency are stored in an Electrically Erasable Programmable Read-Only Memory (EEPROM) inside the sensor housing. The intelligent electronic device (IED) uses this permanent, non-updated, information during the sensor lifetime to correct temperature induced sensitivity changes based on an additional temperature measurement.
Another known solution is called “RCCT calibration through precision potentiometer”. Here the RCCT sensitivity drifts are compensated by re-calibrating and adjusting with a precision potentiometer that is connected to the coil.
Another known solution is called “RCCT calibration on a capacitive discharge rig”. This solution provides a known voltage reference as well as an accurate capacitor in order to extract the sensitivity value. The measurement is based on the measurement of peak heights and the period of the ringdown oscillations. It can call for an offline calibration process.
In any of the aforementioned known calibration methods, a reference current measurement is specified, apart from discharge.
Another known solution is called “RCCT calibration by use of a reference Current Sensor”. It differs from the calibration methods already discussed in how the correction is made. The reference sensor could for example be a shunt or any other sensor type. This option includes the comparison of the Rogowski coil output with the output of a reference CT or coil.
In still another known solution called “RCCT online self calibration with an additional winding combined with dedicated IED,” an additional winding around the Rogowski coil is used to inject a reference current which will induce a reference voltage in the RCCT output signal in addition to the voltage proportional to the derivative of the rated current. This solution is good for self calibration and self diagnostic functions of RCCT+IED. However it calls for an additional winding, which can lead to additional RCCT manufacturing cost.
Among the areas of improvement of the known methods are for example unknown changes of the sensitivity S of Rogowski coils when the environmental conditions are changing, e.g. temperature, mechanical strain, humidity, effects of aging. Improvement with respect to in-factory calibration and diagnosis of the Rogowski coils is specified for implementing these improvements, which also means additional production cost. Another area of improvement can be found in the indication of damaged or out of accuracy class sensors during the coil lifetime, e.g., diagnostic and calibration functions, which is currently not available.